1. Field of the Invention
The present invention relates to an optical voltage.electric field sensor using the Pockels effect for measuring a voltage or electric field applied thereto.
2. Description of the Prior Art
In Japanese Patent Laid-open Publication Nos. 61-223821 and 61 223822, there have been proposed conventional optical voltage.electric field sensors of the type described above, which are designed to avoid influence dependent upon temperature characteristic property of double refraction of a phase plate in the form of a quarter-wavelength plate for enhancing the measurement accuracy thereof. As shown in FIG. 5, the optical voltage.electric field sensor 10 disclosed in the former reference includes a light source 11, a polarizer 12 arranged to receive an incident light beam from light source 11 through a rod lens 19a for dividing the incident light beam into first and second linearly polarized light beams the polarization planes of which are mutually crossed to one another, and a quarter-wavelength plate 13a, a Pockels element 14a, an analyzer 15a and an optical coupler 16 arranged in series on the light transmission path of one of the linearly polarized light beams. The optical voltage.electric field sensor 10 further includes a total reflection mirror 17a arranged to reflect the other polarized light beam, and a quarter-wavelength plate 13b, a Pockels element 14b and an analyzer 15b arranged in series on the light transmission path of the polarized light beam reflected by mirror 17a. The optical voltage.electric field sensor 10 further includes a total reflection mirror 17b arranged to reflect the light beam emanated from the analyzer 15b for transmitting the reflected light beam to the optical coupler 16, a light-receiving element 18a arranged to receive the modulated output light beam from the optical coupler 16 through a rod lens 19b, a pair of amplifiers 19c, 19d for amplifying electric signals applied thereto from the light-receiving element 18a through a distributor 18b, and a divider 18c for dividing the amplified electric signals.
In the optical voltage.electric field sensor 10, it is presumed that absolute values of temperature characteristic property of double refraction in the respective quarter-wavelength plates 13a, 13b are substantially equal to one another. Under such presumption, an optical sum of modulated light beams passed through the Pockels elements 14a, 14b is obtained by the optical coupler 16 to offset influences of the temperature characteristics of double refraction in the wavelength plates 13a and 13b.
However, the former optical voltage.electric field sensor involves the following three problems.
1) Although it is presumed that absolute values of temperature characteristic property of double refraction in the quarter-wavelength plates 13a, 13 b are substantially equal to one another, it is extremely rare that the absolute values of the temperature characteristic property of double refraction become equal to one another in actual practices of the sensor. It is, therefore, unable to completely offset influences of the temperature characteristic property of double refraction even though the modulated output light beam is produced by the optical sum at the optical coupler 16.
2) In the case that the intensities I.sub.1, I.sub.2 of the modulated light beams passed through the Pockels elements 14a, 14b are changed by 1/A, 1/B due to unexpected influences before being applied to the optical coupler 16, the light intensities I.sub.1, I.sub.2 of the modulated light beams and outputs I.sub.1 +I.sub.2 of the divider are calculated as follows: ##EQU1##
It is, therefore, difficult to eliminate errors in measurement of the light intensity caused by the difference between the temperature characteristics of double refraction in the quarter-wavelength plates 13a, 13b.
3) The component parts of the head portion of the sensor increases, and the construction of the sensor becomes complicated and large in size.
As shown in FIG. 6, the optical voltage.electric field sensor 20 disclosed in the latter reference includes a light source 21, a polarizer 22 arranged to receive an incident light beam from the light source 21 through a rod lens 29a for dividing the incident light beam into first and second linearly polarized light beams the polarization planes of which are mutually crossed to one another, and a Pockels element 23a, a quarter-wavelength plate 24a, an analyzer 25a and an optical coupler 26 arranged in series on the light transmission path of one of the linearly polarized light beams. The optical voltage.electric field sensor 20 further includes a total reflection mirror 27a arranged to reflect the other linearly polarized light beam, and a Pockels element 23b, a quarter-wavelength plate 24b, an analyzer 25b and a total reflection mirror 27b arranged in series on the light transmission path of the linearly polarized light beam reflected by mirror 27a. The total reflection mirror 27b is arranged to reflect the modulated incident light beam applied thereto through the quarter-wavelength plate 24b and analyzer 25b for transmitting the reflected light beam to the optical coupler 26. The optical voltage.electric field sensor 20 further includes a light-receiving element 28a arranged to receive the combined output light beams from the optical coupler 26 through a rod lens 29b, a pair of amplifiers 29c, 29d arranged to amplify electric signals applied thereto from the light-receiving element 28a through a distributor 28b, and a divider 28c for dividing the amplified electric signals.
In the optical voltage.electric sensor 20, the crystal axes of the Pockels elements 23a, 23b are arranged to be crossed at 90.degree. about the light axis. With such arrangement of the Pockels elements 23a, 23b, the modulated light beams passed through the Pockels elements 23a, 23b are applied with a phase difference at the quarter-wavelength plates 24a, 24b so that the temperature characteristics of double refraction are reversed in their signs relatively to one another to produce a modulated output light beam wherein influences of the temperature characteristics of double refraction are offset.
However, the latter optical voltage.electric field sensor 20 involves the following problems:
1) Although the crystal axes of the Pockels elements 23a, 23b are arranged to be crossed at 90.degree. about the light axis for offsetting the temperature characteristics of double refraction in the quarter-wavelength plates 23a, 23b, it is very difficult to precisely cross the crystal axes of the Pockels elements 23a, 23b at 90.degree. about the light axis. As a result, a deviation of the angle causes an error in measurement of the intensity of the modulated light beams, and the influences of the temperature characteristics of double refraction in the quarter-wavelength plates 24a, 24b may not be offset even though the modulated output light beam is produced by the optical sum at the optical coupler 26.
2) In case the intensities I.sub.1, I.sub.2 of the modulated light beams are changed by 1/A, 1/B due to unexpected influences before being applied to the optical coupler 26, there will occur errors in measurement of the intensity of the modulated light beams as in the former optical voltage.electric field sensor 10 described above. Moreover, the component parts of the head portion of the sensor increases, and the construction of the sensor becomes complicated and large in size.